Electrochemical sensor and method for manufacturing

ABSTRACT

A sensor includes a sheath that is elongated along a longitudinal axis; a spacer positioned within the sheath and defining first and second channels having lengths that extend along the longitudinal axis; a first elongated member positioned within the first channel; and a second elongated member positioned within the second channel. The first elongated member includes an active surface forming a working electrode and the second elongated member including an active surface defining a counter electrode.

This is a Continuation of U.S. patent application Ser. No. 14/706,123,filed May 7, 2015, now U.S. Pat. No. 9,746,440, which is Continuation ofU.S. patent application Ser. No. 12/674,876, filed May 3, 2011, patentedas U.S. Pat. No. 9,044,178, issued Jun. 2, 2015, which is a NationalStage Application of PCT/US2008/74649, filed Aug. 28, 2008, which claimspriority to the U.S. Provisional Application No. 60/969,071, filed Aug.30, 2007, which applications are incorporated herein by reference. Tothe extent appropriate, a claim of priority is made to each of the abovedisclosed applications.

TECHNICAL FIELD

The present disclosure relates to sensors for measuring bioanalytes andto methods for making such sensors.

BACKGROUND

Electrochemical bio-sensors have been developed for detecting analyteconcentrations in a given fluid sample. For example, U.S. Pat. Nos.5,264,105; 5,356,786; 5,262,035; 5,320,725; and 6,464,849, which arehereby incorporated by reference in their entireties, disclose wiredenzyme sensors for detecting analytes such as lactate or glucose.Technology adapted for enhancing sensor miniaturization, costeffectiveness and durability is desirable.

SUMMARY

One aspect of the present disclosure relates to an electrochemicalsensor including a first elongated member having an active surfacedefining a working electrode, a second elongated member having an activesurface defining a counter electrode and a third elongated member havingan active surface defining a reference electrode. The working electrodeis covered with a sensing layer. The first, second and third elongatedmembers have lengths that extend longitudinally through an insulatorsheath. A spacer member is positioned within the insulator sheath forseparating the first, second and third elongated members. The spacermember defines a first longitudinal channel for receiving the firstelongated member, a second longitudinal channel for receiving the secondelongated member and a third elongate channel for receiving the thirdelongated member. In use, the active surfaces are adapted to contact asample desired to be tested.

Another aspect of the present disclosure relates to an electrochemicalsensor including a first elongated member having an active surfacedefining a working electrode and a second elongated member having anactive surface defining a counter/reference electrode. The workingelectrode is covered with a sensing layer. The first and secondelongated members have lengths that extend longitudinally through aninsulator sheath. A spacer member is positioned within the insulatorsheath for separating the first and second elongated members. The spacermember defines a first longitudinal channel for receiving the firstelongated member and a second longitudinal channel for receiving thesecond elongated member. In use, the active surfaces are adapted tocontact a sample desired to be tested.

A further aspect of the present disclosure relates to an electrochemicalsensor including an elongated member including at least an outer portionthat is conductive. The elongated member is encased within a sheathdefining a plurality of side openings that extend through a thickness ofthe sheath. A sensing material is positioned within the side openings.

A variety of additional inventive aspects will be set forth in thedescription that follows. The inventive aspects can relate to individualfeatures and to combinations of features. It is to be understood thatboth the foregoing general description and the following detaileddescription are exemplary and explanatory only and are not restrictiveof the broad inventive concepts upon which the embodiments disclosedherein are based.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a sensing tip of a first sensorhaving features that are examples of inventive aspects in accordancewith the principles of the present disclosure;

FIG. 2 is an end view showing the sensing tip of the sensor of FIG. 1;

FIG. 3 is a side view of the sensor of FIG. 1;

FIG. 4 is a transverse cross-sectional view of a fiber used within thesensor of FIG. 1 as a working electrode, the fiber is shown covered witha layer of sensing chemistry;

FIG. 5 is a transverse cross-sectional view of a fiber used within thesensor of FIG. 1 as a counter or reference electrode;

FIG. 6 is a schematic view of a sensor system incorporating the sensorof FIG. 1;

FIG. 7 is an end view showing the sensing tip of a second sensor havingfeatures that are examples of inventive aspects in accordance with theprinciples of the present disclosure;

FIG. 8 is a perspective view showing the sensing tip of a third sensorhaving features that are examples of inventive aspects in accordancewith the principles of the present disclosure;

FIG. 9 is a perspective view of a working electrode configurationsuitable for use in embodiments in accordance with the principles of thepresent disclosure;

FIG. 10 is an end view of the working electrode configuration of FIG. 9;

FIG. 11 is a side view of the working electrode configuration of FIG. 9;

FIG. 12 is a cross-sectional view taken along section line 12-12 of FIG.11 showing a first sensing chemistry configuration; and

FIG. 13 is a cross-sectional view taken along section line 12-12 of FIG.11 showing a second sensing chemistry configuration.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary aspects of the presentdisclosure which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

The following definitions are provided for terms used herein:

A “working electrode” is an electrode at which the analyte (or a secondcompound whose level depends on the level of the analyte) iselectrooxidized or electroreduced with or without the agency of anelectron transfer agent.

A “reference electrode” is an electrode used in measuring the potentialof the working electrode. The reference electrode should have agenerally constant electrochemical potential as long as no current flowsthrough it. As used herein. the term “reference electrode” includespseudo-reference electrodes. In the context of the disclosure, the term“reference electrode” can include reference electrodes which alsofunction as counter electrodes (i.e., a counter/reference electrode).

A “counter electrode” refers to an electrode paired with a workingelectrode to form an electrochemical cell. In use, electrical currentpasses through the working and counter electrodes. The electricalcurrent passing through the counter electrode is equal in magnitude andopposite in sign to the current passing through the working electrode.In the context of the disclosure, the term “counter electrode” caninclude counter electrodes which also function as reference electrodes(i.e., a counter/reference electrode).

A “counter/reference electrode” is an electrode that functions as both acounter electrode and a reference electrode.

An “electrochemical sensing system” is a system configured to detect thepresence and/or measure the level of an analyte in a sample viaelectrochemical oxidation and reduction reactions on the sensor. Thesereactions are transduced to an electrical signal that can be correlatedto an amount, concentration, or level of an analyte in the sample.Further details about electrochemical sensing systems, workingelectrodes, counter electrodes and reference electrodes can be found atU.S. Pat. No. 6,560,471 that is hereby incorporated by reference in itsentirety.

“Electrolysis” is the electrooxidation or electroreduction of a compoundeither directly at an electrode or via one or more electron transferagents.

An “electron transfer agent” is a compound that carries electronsbetween the analyte and the working electrode, either directly, or incooperation with other electron transfer agents. One example of anelectron transfer agent is a redox mediator.

A “sensing layer” is a component of the sensor which includesconstituents that facilitate the electrolysis of the analyte. Thesensing layer may include constituents such as an electron transferagent, a catalyst which catalyzes a reaction of the analyte to produce aresponse at the electrode, or both. In some embodiments, the sensinglayer has a generally dry or non-hydrated state prior to use. In suchembodiments, the sensing layer can be hydrated during use by waterwithin the fluid sample being tested.

FIGS. 1-3 illustrate a sensor 20 having features that are examples ofinventive aspects in accordance with the principles of the presentdisclosure. The sensor 20 includes first, second and third elongatedmembers 21, 22 and 23 each having at least a portion that iselectrically conductive. An insulating layer 26 surrounds the first,second and third elongated members 21, 22 and 23. The sensor 20 includesa sensing tip 28 at which active surfaces 21 a, 22 a and 23 a of thefirst, second and third elongated members 21, 22 and 23 project beyondthe insulating layer 26. The first elongated member 21 functions as aworking electrode and includes a sensing layer 24 (see FIG. 4) that ispositioned at least at the active end of the first elongated member 21.The second and third elongated members 22, 23 function respectively ascounter and reference electrodes. The sensor 20 also includes a base end29 positioned opposite from the sensing tip 28. The elongated members21-23 can extend from the sensing tip 28 to the base end 29. A length Lof the sensor 20 extends from the sensing tip 28 to the base end 29. Inone embodiment, the length L is in the range of 20-40 centimeters.

Referring to FIGS. 1 and 3, the sensor 20 includes a spacer 30positioned within the insulating layer 26 for separating the elongatedmembers 21-23. The spacer is preferably constructed of a dielectricmaterial and includes first, second and third channels 31-33 that areparallel and that extend through along a longitudinal axis 34 of thesensor generally from the sensing tip 28 to the base end 29. The first,second and third channels 31-33 respectively receive the first, secondand third elongated members 21-23. As shown at FIG. 1, distal ends ofthe first, second and third elongated members 21-23 are generally flushwith a distal end 35 of the spacer 30. In other embodiments, the distalends of the elongated members 21-23 project outwardly beyond the distalend 35 of the spacer 30. In one embodiment, the spacer has an extrudedpolymer construction.

Referring to FIG. 1, the active surfaces 21 a, 22 a, and 23 a of theelongated members 21-23 face radially outwardly from the channels 31-33and are exposed (i.e., not covered by the spacer 30). Radially inwardlyfacing surfaces 21 b, 22 b and 23 b of the elongated members 21-23 arecovered by the spacer 30. The insulating layer 26 covers the radiallyoutwardly facing portions of the elongated members 21-23 that do notproject beyond the insulating layer 26. In this way, the insulatinglayer 26 and the spacer 30 control the sizes of the active surfaces 21a, 22 a and 23 a. In other words, the insulating layer 26 and the spacer30 are configured to ensure that only the active surfaces 21 a, 22 a and23 a are exposed to an analyte desired to be sensed within a testsample.

FIG. 6 illustrates an electrochemical sensing system 40 thatincorporates the sensor 20 of FIGS. 1-3. The working electrode of theelongated member 21 is electrically connected to a wire 41 by aconnector or hub 42 positioned at the base end 29 of the sensor 20. Thewire 41 electrically connects the working electrode of the elongatedmember 21 to a controller 46. The controller 46 can be any type ofcontroller such as a micro-controller, a mechanical controller, asoftware driven controller, a hardware driven controller, a firmwaredriven controller, etc. The controller can include a microprocessor thatinterfaces with memory. The hub 42 also electrically connects the secondand third elongated members 22, 23 to wires 47, 48 routed to thecontroller 46. In this way, the reference and counter electrodes arealso electrically connected to the controller 46.

In use of the sensing system 40, the sensing tip 28 of the sensor 20 isimmersed within a test volume 50 of a liquid sample (e.g., a bloodsample) containing an analyte desired to be sensed. The sample may be anex vivo or in vivo sample. With the sensor 20 so positioned, waterwithin the test volume 50 can diffuse into the sensing layer 24 t theactive tip of the working electrode such that the sensing layer 24 ishydrated. The analyte within the test volume 50 also diffuses into thesensing layer 24. A voltage potential is then applied between thecounter electrode (i.e., the second elongated member 22) and the workingelectrode (i.e., the first elongated member 21). The reference electrode(i.e., the third elongated member 23) assists in measuring the voltagepotential between the working and counter electrodes. When the potentialis applied, an electrical current will flow through the test volume 50between the counter electrode and the working electrode. The current isa result of the oxidation or reduction of the analyte in the test volume50. This electrochemical reaction occurs via the electron transfer agentin the sensing layer 24 and the optional electron transfercatalyst/enzyme in the sensing layer 24. By measuring the current flowgenerated at a given potential, the concentration of a given analyte inthe test sample can be determined. Those skilled in the art willrecognize that current measurements can be obtained by a variety oftechniques including, among other things, coulometric, potentiometric,amperometric, voltammetric, and other electrochemical techniques.

In certain embodiments, the elongated members 21-23 of the sensor 20 caneach include an electrically conductive wire or fiber. For example, theelongated members 21-23 can each include a metal wire or a glassy carbonfiber. In a preferred embodiment, the elongated members 21-23 have acomposite structure and each includes a fiber having a dielectric core60 surrounded by a conductive layer 62. In the case of the firstelongated member 21, the conductive layer 62 is covered by the sensinglayer 24 (see FIG. 4) and functions as a working electrode. In the caseof the second and third elongated members 22, 23 (see FIG. 5 which isrepresentative of both the first and second elongated members 22, 23),the conductive layers 62 are not covered with a sensing layer. For theelongated members 22, 23, the conductive layers 62 function as thecounter and reference electrodes, respectively.

A preferred composite fiber for constructing the elongated members 21-23is sold under the name Resistat® by Shakespeare Conductive Fibers LLC.This composite fiber includes a composite nylon monofilament conductivethread material made conductive by the suffusion of about a 1 micronlayer of carbonized nylon isomer onto a dielectric nylon core material.The Resistat® material is comprised of isomers of nylon to create thebasic 2 layer composite thread. However, many other polymers areavailable for the construction such as: polyethylene terephthalate,nylon 6, nylon 6,6, cellulose, polypropylene cellulose acetate,polyacrylonitrile and copolymers of polyacrylonitrile for a firstcomponent and polymers such as of polyethylene terephthalate, nylon 6,nylon 6,6, cellulose, polypropylene cellulose acetate, polyacrylonitrileand copolymers of polyacrylonitrile as constituents of a secondcomponent. Inherently conductive polymers (ICP) such as dopedpolyanaline or polypyrolle can be incorporated into the conductive layeralong with the carbon to complete the formulation. In certainembodiments, the ICP can be used as the electrode surface alone or inconjunction with carbon. The Resistat® fiber product is currently soldwith a circular transverse cross-sectional profile. By post forming orextruding the Resistat® fiber, other transverse cross-sectional profiles(e.g., generally triangular) can be provided. The Resistat® fiber isavailability in diameters of 0.0025 to 0.016 inches, which as suitablefor sensors in accordance with the principles of the present disclosure.Example patents disclosing composite fibers suitable for use inpracticing sensors in accordance with the principles of the presentdisclosure include U.S. Pat. Nos. 3,823,035; 4,255,487; 4,545,835 and4,704,311, which are incorporated herein by reference.

The sensing layer 24 preferably includes a sensing chemistry such as aredox compound or mediator. The term redox compound is used herein tomean a compound that can be oxidized or reduced. Exemplary redoxcompounds include transition metal complexes with organic ligands.Preferred redox compounds/mediators are osmium transition metalcomplexes with one or more ligands having a nitrogen containingheterocycle such as 2,2′-bipyridine. The sensing material can alsoinclude a redox enzyme. A redox enzyme is an enzyme that catalyzes anoxidation or reduction of an analyte. For example, a glucose oxidase orglucose dehydrogenase can be used when the analyte is glucose. Also, alactate oxidase or lactate dehydrogenase fills this role when theanalyte is lactate. In systems such as the one being described, theseenzymes catalyze the electrolysis of an analyte by transferringelectrons between the analyte and the electrode via the redox compound.Further information regarding sensing chemistry can be found at U.S.Pat. Nos. 5,264,105; 5,356,786; 5,262,035; and 5,320,725, which werepreviously incorporated by reference in their entireties.

The insulating layer 26 of the sensor 20 preferably serves numerousfunctions to the sensor 20. For example, the insulating layer 26preferably electrically insulates the elongated members 21-23.Additionally, the insulating layer 26 preferably provides mechanicalstrength for protecting the elongated members 21-23. In one nonlimitingembodiment, the insulating layer 26 is made of a polymeric material suchas polyimide, polyurethane or other materials. In certain embodiments,the insulating layer 26 can have a maximum outer dimension (e.g., anouter diameter) less than 2 millimeters. The insulating layer 26 may bean off shelf shrink component, a co-extruded, extruded or injectionmolded component serving the purposes of defining the available surfacearea of the electrodes accessible by the analyte and insulating theinactive portion of the elongated members in order for it to provide aconductive link from the active portions of the elongated members backto the hub 42 or to another structure such as an alternative type ofterminal connector, a display or a wireless node.

It will be appreciated that the sensor 20 can be used for ex vivo or invivo applications. In certain embodiments, the sensor 20 can beincorporated into a peripheral catheter to provide on-line monitoring ofbioanalytes in the same manner described in U.S. Pat. No. 6,464,849,which was previously incorporated by reference herein.

FIG. 7 shows a second sensor 120 having features that are examples ofinventive aspects in accordance with the principles of the presentdisclosure. The sensor 120 has the same configuration as the sensor ofFIGS. 1-4, except a combined counter electrode 148 has been used inplace of separate reference and counter electrodes. In certainembodiments, the counter electrode 148 can function only as a counterelectrode or as a counter/reference electrode. Also, the sensor 120 hasa spacer 140 that has been modified to include only two channels insteadof three. The counter/reference electrode 148 can include a materialsuch as silver silver-chloride. The silver silver-chloride can be coatedor otherwise provided about the surface of a conductive member such as awire or fiber (e.g., a composite fiber as described above). In oneembodiment, the silver silver-chloride can be incorporated into an outerlayer of the wire or fiber.

FIG. 8 shows a third sensor 220 having features that are examples ofinventive aspects in accordance with the principles of the presentdisclosure. The sensor 220 has the same construction as the sensor 20 ofFIGS. 1-4, except the insulating layer 26 has been extended at thesensing tip 28 to cover and provide mechanical protection for the activeends of the elongated members 21-23. The end of the insulating layer 26is open to allow a test sample to access the active ends of theelongated members 21-23. The spacer can be use to support the insulatinglayer 26 and to provide open space/void between the insulating layer 26and the active surfaces of the elongated members 21-23 such that testfluid can readily reach the active surfaces without obstruction from thesheath 26. For example sufficient space is provided adjacent the activesurface of the working electrode to allow the sensing chemistry torapidly hydrate and to allow analyte desired to be sensed to diffuseinto the sensing chemistry.

FIGS. 9-11 show an elongated member 221 having features that areexamples of inventive aspects in accordance with the principles so ofthe present disclosure. It will be appreciated that the elongated member221 can be used as a working electrode in any of the above identifiedembodiments, or in any other type of electro-chemical sensor. Theelongated member 221 includes a conductive member 219 (e.g., aconductive wire or fiber as described above) having at least an outersurface or portion that is electrically conductive. In a preferredembodiment, the conductive member 219 has the same construction as theResistat® composite fiber and includes a dielectric core 60 surroundedby a conductive layer 62. The elongated member 221 also includes asheath 250 that surrounds the conductive member 219 and extends alongthe length of the conductive member 219. The sheath 250 defines athickness t that extends from and inner boundary 251 (e.g., an innerdiameter) of the sheath 250 to an outer boundary 253 (e.g., an outerdiameter) of the sheath 250. The inner boundary 251 includes an innersurface that engages the outer conductive layer 62 of the conductivemember 219. In one embodiment, the inner surface of the sheath 250 isconfigured to prevent a test sample from moving along the length of theelongated member in the region between the sheath 250 and the conductivemember 219. The sheath 250 can also be referred to as an insulatinglayer, a membrane, a porous layer, a micro-porous layer, or like terms.

Referring still to FIGS. 9-11, the sheath 250 defines a plurality ofside openings 260 (i.e., cells, pores, etc.) that extend through thesheath 250 from the inner boundary 251 to the outer boundary 253. In oneembodiment, the side openings 260 have central axes 261 that extend in aradial direction relative to a central longitudinal axis 263 of theelongated member 221. In the depicted embodiment, the openings aregenerally cylindrical in shape. In other embodiments, other openingshapes can be used. In certain embodiments, the openings can havediameters less than or equal to about 100 microns, depths less than orequal to about 100 microns (as defined by the thickness t of thesheath). It will be appreciated that the drawings are schematic and thatthe number, concentration and size of the openings 260 are forillustration purposes only and are not to scale.

Sensing chemistry 224, such as the sensing chemistry described above, isprovided within each of the openings 260. The sensing chemistry 224within each of the openings 260 is electrically connected to theconductive layer 62 of the elongated member 219.

The openings 260 containing sensing chemistry 224 provide separateworking electrode cells that are all electrically connected to theconductive member 219. During testing of a sample, the sensing chemistryis hydrated and a voltage is applied between the working electrode cellsand a counter electrode. When the voltage potential is applied, theanalyte being sensed reacts with the sensing chemistry at the workingelectrode cells, and the working electrode cells provide outputs thatare combined at the conductive member 219 to cause electrical current toflow through the conductive member 219. This architecture isparticularly advantageous when adapted to sensor fabrication related toenzyme sensor chemistries typical to glucose, lactate and othermedically important analytes. The architecture allows in-situfabrication of mass arrayed sensors directly on the surface of canulaeor guidewires and makes possible “smart” syringes and various similardevices. It is primarily beneficial to applications requiringimplantation, greater micro-miniaturization, or remote placement, andprovides manufacturing advantages of improved process, increasedthroughput, reduced product defects, simplified quality control andquality assurance, and simplification of manufacturing complexity. Thearchitecture also provides mechanical protection of the sensingchemistry and allows for the amount of sensing chemistry exposed toanalyte to be accurately controlled. The architecture may also beemployed in advanced test strip designs for applications such as glucosemonitoring where multiple sensors may be arrayed on single cards orrotating cylinders eliminating the need for patient handling ofindividual sensor strips for each test. These sensors can use a fractionof the space needed for similar “bandolier” type designs currently onthe market and likely produced with lower reject ratios and cost than isnow available.

FIG. 12 shows an embodiment where the sensing chemistry 224 forms solidrods 224 a within the openings 260. In the depicted embodiment, the rods224 a extend outwardly from the conductive member 219 in a radialdirection for the full depth of the openings 260 (i.e., for the fullthickness of the sheath 250). In other embodiments, the rods 224 a canextend outwardly from the conductive member 219 in a radial directionfor a partial depth of the openings 260.

For the embodiment of FIG. 12, the sheath 250 can be made of acompletely dielectric material such as polycarbonate or other dielectricmaterials. Alternatively, for the embodiment of FIG. 12, the sheath 250can also include both dielectric and electrically conductive portions.For example, the sheath 250 can include an inner core portion that isdielectric, and a surface layer that is electrically conductive. Theconductive surface layer can include the surface defining the innerboundary 251, the surface defining the outer boundary 253, and thesurfaces defining the openings 260. Inherently conductive polymers (ICP)such as doped polyanaline or polyprolle could be used to form theconductive surface layers of the sheath. Example ICP preparationprocesses are disclosed at U.S. Pat. No. 5,849,415, which is herebyincorporated by reference in its entirety.

FIG. 13 shows an embodiment where the sensing chemistry 224 forms hollowstructures 224 b (e.g., hollow tubes or cylinders) that line theopenings 260. The hollow structures 224 b include passages 270 alignedalong the central axes 261 of the openings. The hollow structures 224 bare electrically connected to the conductive member 219 and extendradially outwardly from the conductive member 219 for at least a portionof the depth of each opening 260. For this embodiment, the sheath 250preferably includes an inner core portion that is dielectric, and asurface layer that is electrically conductive as described above. Forexample, it is preferred for the openings 260 to be lined by aconductive layer that surrounds the hollow structures 224 b and thatprovides an electrical pathway for carrying current radially through thesheath 250 between the conductive member 219 and the sensing chemistry224.

The passages 270 assist in rapidly hydrating the sensing chemistry 224to improved sensor response times. Also, the size (e.g., the diameter)of the passages can be selected to extend sensor life bycontrolling/limiting the amount of sensor chemistry that can react withthe analyte desired to be sensed at a given time. For example, while thepassages 270 can be sized to allow the sensing chemistry to be rapidlyhydrated, they can also be sized to allow the analyte being sensed toreact only with the outermost sensing chemistry. In one embodiment, thepassages have inner diameters less than 100 microns. For example, as theanalyte enters the passages 270, the analyte reacts with the portion ofthe hollow structure 224 b located furthest from the conductive member219 and is consumed before the analyte can migrate further radiallyinwardly into the passages 270. Over time, the portion of the hollowstructure 224 b that is furthest from the conductive member 219 isdepleted. When this occurs, the analyte is able to move deeper radiallyinto the openings 260 to reach active portions of the hollow structures260. This process continues until the analyte depletes the entire radiallengths of the hollow structures 224 b and the life of the sensor ends.In this progression, the portions of the hollow structures locatedclosest to the conductive member 219 are the last portions of thesensing chemistry to be depleted.

Two manufacturing principles are preferably taken into considerationwhen forming the openings 260 in the sheath 250. The first is theability to easily create large arrays of smooth straight pores through aconformable polymer and the second is the ability to regulate both thedensity of the pore field and the ratio of the pore opening to itsdepth. To fabricate an elongated member such as the elongated member221, an elongate conductive member (as described above) is coated alongsome portion of its length with a thin dielectric film suitable for deepreaction ion etching (DRIE) or other process capable of forming suitableblind passages corresponding to the openings 260.

A first example of a process suitable for the creating the dielectriccoating for the sheath is to produce a thin film of Parylene polymerover some portion of the conductive member's length to a thickness ofpreferably less than ˜80 microns. The Parylene coating may be appliedusing standard commercially available methods whereby the solid phasemonomer precursor (monomeric diradical para-xylylene) is heated in avacuum chamber to produce a vapor. The vapor is next drawn into acoating chamber where the conductive member is exposed to the vapor thatcondenses uniformly over the outer surface of the conductive member. Asused in the coating step here, the vacuum chamber can include air tightentrance and exit glands allowing for a continuous passing of theconductive member into the chamber target area to produce the desireddielectric film along its desired length. Parylene conformal coatingsare suitable to DRIE, have relatively low temperature processrequirements, and can be controlled in uniform thicknesses from 500Angstroms to 75 microns to provide consistent, defect free coatings overany geometry without sagging. These characteristics are particularlyadvantageous to applying coatings over elongate, non-planer (e.g.,cylindrical) substrates.

The minimal negative impacts of the Parylene coating process on targetsubstrates expands the potential for use of many standard commercialgrade monofilament polymer threads as conductive members for biosensorapplications. For most of these materials a “sputter” coating step maybe employed to produce a thin film conductive surface on themonofilament from a noble metal such as gold or platinum rather than thesuffusion coating method described above. The resultant metallic thinfilm is sufficient to provide a suitable substrate for the Parylenecoating and act as the conductor. Sputter coating provides the option ofspecifying various metals and alloys better suited to alternativesensing chemistries. Where sputter coating is desired, the two steps ofsputter coating and Parylene coating as well as certain subsequentcoatings having such purpose as to pattern electrically differentiatedzones for such use as integrated counter/reference electrodes (e.g.,counter/reference electrode coating provided at the outer surface of thesheath 250), may be integrated as part of a single continuousmanufacturing line in much the same way that insulated co-axialelectrical wires are commonly fabricated.

A second example of a potentially suitable process for integrating thesheath 250 onto the conductive member is to dissolve a polycarbonate orpolyester resin precursor in a volatile solvent specific to the resinsuch as methylene chloride, but such solvent remaining harmless to thebase material of the conductive member—such as carbon suffused or goldsputter coated nylon discussed earlier that has been specificallytreated to accept the particular over-coating uniformly. The resinsolute then may be used as a coating bath for a first process stepwhereby the conductive member travels continuously into one or morebaths and is then drawn out vertically at such rate and under suchconditions so as to evaporate the solvent between each bath and cause athin film (e.g., less than 80 microns) of polycarbonate to remain boundto the surface of the conductive member. Recent developments using dopedICP co-polymers to produce aqueous cast films and coatings nowcommercially available (Panipol, Inc., Cornelius) suggest that suchfilms may be coated directly onto the nylon substrate monofilamenteither by this process or by passing through an atomized fluid bath

Regardless of the particulars of the dielectric coating, the coatedconductive member may then be subjected to a next process generallysimilar to that of DRIE fabrication for flat membranes, except that thecoated conductive wire filament sensor substrate is drawn through, orenclosed within, a radiation containment chamber so configured as toexpose a portion of the wire circumference to the directed high energyradiation source typical to track etched pore fabrication. The directedenergetic particles within the chamber pass fully through the dielectriccoating material leaving straight tracks of damaged molecular debris inthe dielectric film that are mostly absorbed at the carbonized nylon (ormetallic) conductive surface of the conductive member. The damagedmaterial may be then be selectively etched away using another reel toreel process potentially augmented with ultrasonic stirring to createthe arrayed analysis cells extending out from the conductive face of theconductive member to the outer distal surface of the dielectric thinfilm.

After the porous coating has been cleaned and the surface energiesreduced by surfactants or corona discharge treatment, the coatedconductive member is collected on reels and sent through the chemistrycoating station. In this process, the coating station may be eitherpressurized or put under high vacuum and cooled to near the freezingpoint of the sensor chemistry in order to facilitate the filling of themicro-sensor array while in the coater. In some embodiments, the sensorchemistry can fill the individual cells completely before exiting thecoating station and the excess chemistry wiped clean of the exposedouter surface of the dielectric coating by a sealing gland upon exit.The conductive member then enters the curing station where partialvacuum and/or reduced humidity and slightly elevated temperature causethe volatile components of the sensor chemistry to escape to atmosphere.The sensor chemistry should either be formulated to dry within the cellwith the residue substantially filling the void after curing or be driedto a thin film coating the walls of the cell, but leaving an openchannel concentric to the pore diameter, passing substantially to thebottom of the cell at the conductor interface to facilitate hydration.

If it is desired to provide the sheath 250 with a conductive surface, asdescribed above, additional processing steps preceding the sensorchemistry deposition can be used to create a secondary conductive layerat least lining the inner wall of the analysis cell. The preferredmethod would use the ICP co-polymer aqueous solution to cast the DRIEprocess-able porous layer which would yield cells inherentlysemi-conductive and bound electrically to the highly conductive outputconnector surface of the monofilament. Alternatively, one of severalavailable ICP preparation processes (see U.S. Pat. No. 5,849,415)combining the steps of chemically functionalizing the porous substrateand creating molecular attachments to the ICP polymer would be employedto create the required conductive surface domain receptive to thesensing chemistry. Once created and filled with sensor chemistry theindividual nano analysis cells would then function as working electrodeswith the thickness of the porous substrate in part determining theservice life of the sensor. A non ICP coating process for plastics usingan aqueous suspension of carbon nanotubes has been developed by EikosInc. referred to as Nanoshield™. This process offers similar thin filmconductive properties. Relatively high conductivities are reported forthe Nanoshield process providing surface films without substantialdimensional changes to the substrate dimension.

Following the required coating and curing steps specific to the agentsselected, the sensors fabricated using the in situ process of porouslayer fabrication are functionally complete and do not need any outerprotective component. For most in-vivo applications however, a finalbiocompatible layer can be used to provide an anti thrombogenic surfaceto protect the micro-array field from the body's damaging immuneresponses. Following this process the completed sensor may enter anynumber of secondary operations where it can be configured for usedirectly in catheters, syringes, or tissue as needed by the specificapplication.

Many prior art enzymatic sensor designs considered as candidates forminimally invasive diagnostics suffer from trade-offs made inminiaturizing the structure adequately for non-surgical short termimplantation. Despite their proven advantages of low cost, andrelatively simple function that are important characteristics needed forall disposable diagnostics, the use of organic sensing chemistry andhistorical process techniques dependent on flat structures makeachieving the goal of highly repeatable and reliable sensors that arealso minimally invasive elusive. Enzymatic sensors typically require alayered structure that generally includes an entry port, some means tomove the analyte into the sensor and a central chamber having the sensorchemistry where the analysis is done. The high precision required inmaintaining an accurate and repeatable relationship among the functionalelements of the device is dependent on processes such as screenprinting, die cutting and droplet deposition that although are easilyachieved for in-vitro devices often limit the degree to which thesesensors can be micro-miniaturized.

The demand for smaller and smaller disposable sensors capable ofresiding in tissue, organs or vasculature without interference to thebiological process or discomfort to the patient is likely to drive theflat sensor design beyond the practical limit of the most cost effectivemanufacturing processes. Paradoxically, while the theoretical size ofthe active analysis surface needed to produce a useful electrical signalhas continued to shrink to the point where a practical enzymatic glucoseanalysis can occur on the tip of a human hair ˜80 microns diameter, thetypical planar construction package based on laminar films and able tosupport that analysis has a minimum single surface dimension greaterthan 1000 microns, most of which is required for tolerance allowancesand bond pads. Certain inventive aspects disclosed herein are adapted toprovide new structures that can leverage the full miniaturizationpotential of enzymatic sensors while providing both significantly lowercosts and the high reliability demanded by implantable diagnostics.

In certain embodiments in accordance with the principle so of thepresent disclosure, a DRIE active film is cast in-situ on the conductiveoutput surface of a conductive member along with the etched pathways ofa pore field forming mass arrayed nano analysis cells. In function, thetarget analyte flux diffuses into each cell and is converted by theresident sensor chemistry to a proportional current collected at thefloor of the cell by the common conductive output of the conductivemember. The design and control of the DRIE film thickness, the densityof the pore field, the ratio of the pore aperture to the area of itsreactive surface within the analysis cell, and the surface energy of itswalls are all highly predictable. Such abilities may be used in thefabrication of biochemical sensors to affect a wide range of sensorresponse characteristics including: life cycle, sensitivity, rise time,hydration time, and interferent response becoming inherent properties ofthe cast film rather than as independent external variables. Thiscontrol is predicted by the highly ordered and relatively staticenvironment of the sensor chemistry (e.g., hydrogel) within the nanoanalysis cells that is not possible to achieve in the dynamic macroenvironment of assembled laminar membranes and conductors separated bygel matrices typical to planar enzymatic sensor technology. In additionto controlling the physical characteristics of the analysis cells, thesensor chemistry physical properties may also be optimized to controlparameters such as viscosity, residual moisture, cured film thickness,etc. and then related to the specific nano cell properties to furtherdefine performance and coating integrity. By creating a fixed andintegral membrane, cast in place on the conductive member, the sensorbecomes a unified structure more robust and with greater reliability andreduced susceptibility to issues such as mechanically induced electronicnoise resulting from the dynamic environment of living tissue. Forcertain embodiments of the present disclose, the potential forminiaturization down to diameters<200 microns is achievable for wiredenzyme type diagnostic sensors and it may be anticipated that enzymaticbiosensor constructions can be fabricated for commercial use that willbe within the dimensional footprint of competitive solid state andoptical sensors at substantially lower market entry costs. Biosensorshaving these new capabilities that are producible at high volume and lowcost may allow the development of a new generation of continuous sensorsfor minimally invasive clinical diagnosis and monitoring of relevantbiomarkers such as glucose, lactate and other medically significantmolecules in humans.

From the foregoing detailed description, it will be evident thatmodifications and variations can be made without departing from thespirit or scope of the broad inventive aspects embodied in theembodiments disclosed herein. For example, any of the embodimentsdisclosed herein can use separate reference and counter electrodesinstead of combined counter/reference electrodes.

What is claimed is:
 1. An electro-chemical sensing device comprising: aconductive member having an outer surface; a sheath covering theconductive member, the sheath having a thickness defined between innerand outer boundaries of the sheath, the inner boundary of the sheathbeing located at the outer surface of the conductive member, the sheathdefining a plurality of side openings that extend through the thicknessof the sheath from the inner boundary to the outer boundary; and sensingchemistry positioned within the plurality of side openings so that thesensing chemistry extends radially along the thickness of the sheath,wherein the sensing chemistry within each side opening includes hollowstructures lining the side opening, the hollow structures beingelectrically connected to the conductive member, wherein each sideopening of the sheath is lined by a conductive layer surrounding thehollow structure to provide an electrical pathway for carrying currentradially through the sheath between the conductive member and thesensing chemistry.
 2. The electro-chemical sensing device of claim 1,wherein the conductive member includes a dielectric core surrounded by aconductive layer.
 3. The electro-chemical sensing device of claim 1,wherein the inner boundary of the sheath is configured to inhibit a testsample from moving along a length of the conductive member in a regionbetween the sheath and the conductive member.
 4. The electro-chemicalsensing device of claim 1, wherein the side openings are generallycylindrical in shape.
 5. The electro-chemical sensing device of claim 1,wherein the side openings have diameters less than about 100 microns. 6.The electro-chemical sensing device of claim 1, wherein the sensingchemistry within each side opening is electrically connected to theconductive member.
 7. The electro-chemical sensing device of claim 1,wherein the sensing chemistry within each opening includes solid rodsextending outwardly from the conductive member in a radial direction atleast partially through the opening.
 8. The electro-chemical sensingdevice of claim 1, wherein the sheath is formed at least partially fromdielectric material.
 9. The electro-chemical sensing device of claim 8,wherein the sheath includes an inner dielectric layer surrounded by anelectrically conductive surface layer.
 10. An electro-chemical sensingdevice comprising: a conductive member having an outer surface; a sheathcovering the conductive member, the sheath having a thickness definedbetween inner and outer boundaries of the sheath, the inner boundary ofthe sheath being located at the outer surface of the conductive member,the sheath defining a plurality of side openings that extend through thethickness of the sheath from the inner boundary to the outer boundary;and sensing chemistry positioned within the plurality of side openings,wherein the sensing chemistry within each side opening includes hollowstructures lining the side opening, the hollow structures beingelectrically connected to the conductive member, wherein each sideopening of the sheath is lined by a conductive layer surrounding thehollow structure to provide an electrical pathway for carrying currentradially through the sheath between the conductive member and thesensing chemistry.